1. Field of the Invention
The present invention relates to apparatus for controlling the output of an excimer laser device which excites laser gases by pulse discharge to oscillate laser beam pulses and which is used mainly as a light source of a successively moving type reduction projection aligner (hereinafter referred to as "stepper").
2. Description of the Related Art
A stepper requires a strict exposure control to secure high resolution in forming circuit patterns. An excimer laser used as a light source of the stepper is a laser device of a type which excites laser gases by pulse discharge so as to output laser beam pulses. It is observed that the energy of a laser beam pulse (hereinafter called "pulse energy") output from conventional laser devices varies for each pulse. To accurately control the exposure by the laser beam pulse, it is necessary to greatly reduce the variance of the pulse energy for each pulse. Furthermore, since light emitted from the excimer laser device is not continuous, the control of the exposure quantity is different from conventional shutter control method used in controlling continuous light emitted from a mercury lamp.
"Excimer Laser Lithography" by Miyaji et al., International Laser/Application '91, Seminar L-5, pp.38-51, discloses a method to improve the accuracy of exposure quantity control using a multiple laser beam pulse exposure by successively oscillating a plurality of laser beam pulses.
This method utilizes the fact that variance of the pulse energy for a laser beam pulse oscillated by the excimer laser device is approximated by the normal distribution and therefore the variance of the accumulated pulse energy for n laser beam pulses is equal to 1/n.sup.1/2.times. of the variance of the pulse energy for one laser beam pulse. Thus, the number of pulses N required to achieve control accuracy A of exposure quantity is given by: EQU N.gtoreq.{(.DELTA.P/P)/A}.sup.2
where .DELTA.P/P is a variance of the pulse energy for one laser beam pulse.
For example, if a variance of the energy for one pulse .DELTA.P/P is 15% (3 .sigma.) and the required accuracy of exposure quantity control A is 1.5% (3 .sigma.), then N.gtoreq.100. Thus, a desired control accuracy can be achieved by repeating the successive pulse oscillation 100 or more times.
In the stepper, light exposure and stage movement are alternately performed. Thus, the excimer laser used for a light source of the stepper is operated in burst mode. The burst mode is a mode which repeats the operation that a predetermined number of laser beam pulses are successively oscillated and then the laser pulse oscillation is suspended for a predetermined interval of time. In other words, a short-time of successive pulse oscillation period and a short oscillation suspension period are alternately repeated in the burst mode.
In this specification, the phrases, "successive pulses" and "successive pulse oscillation" are used to mean that pulse discharge is repeated to generate a plurality of successive laser beam pulses. Thus, these phrase are used in a sense different from what "successive oscillation laser" and "CW oscillation" are generally referred to.
As described above, since the excimer laser is a pulse discharge/excitement gas laser, it is difficult to maintain oscillation of laser beam pulses having a constant pulse energy. This is because the density disturbance of the laser gas occurs in a discharge space due to the discharging. Moreover, the density disturbance makes subsequent discharge uneven or unstable, which causes a temperature rise at a part of the discharge electrode surface, which in turn makes the subsequent discharge further deteriorated, uneven and unstable. This tendency is especially noticeable at the initial stage of the successive pulse oscillation. Although stable discharge and relatively high pulse energy are obtained in the first pulse directly after the oscillation suspension period, so-called spiking phenomenon appears thereafter in which the discharge is deteriorated and the pulse energy is gradually reduced. This phenomenon is shown by B in FIG. 16.
As describe above, the excimer laser device operating in the burst mode has the problem that variance of the energy for each pulse reduces the accuracy of exposure quantity control and the spiking greatly increases the variance of the pulse energy to thereby further reduces the accuracy of the exposure quantity control.
Recent improvement in the sensitivity of photosensitive materials coated on wafers makes it possible to achieve exposure with a reduced number of successive laser beam pulses. That is, there is a tendency that the number of pulses to be used for exposure decreases.
As the number of pulses decreases, however, variance of pulse energy increases, making it further difficult to maintain the accuracy of exposure quantity control by using only the above-described multiple pulse exposure control method. Therefore, reduction in the variance of the pulse energy and especially elimination of the influence by the spiking in the burst mode have been desired.